The electric and magnetic field vectors of electromagnetic radiation are perpendicular to each other and to the direction of propagation. In polarized light, the distribution of these vectors is not arbitrary, but ordered rather. Linear polarization is characterized by a constant direction of the electric field vector. Circular polarization is present when the electric field vector has a constant length and rotates around the direction of propagation. These are special cases of elliptical polarization, in which the terminal point of the electric field vector describes an ellipse in any fixed plane that is normal to and intersects the direction of propagation.
The signal-to-noise ratio of a photodiode can be enhanced if only certain states of polarization are detected. This is especially favorable when ambient light, in particular bright sunlight, interferes with the reception of a light signal to be detected and the signal has a lower intensity than the ambient light.
Polarizing filters are used in cameras to reduce oblique reflections from non-metallic surfaces, in particular light reflected from the ground. Time-of-flight sensors emit a polarized laser beam, which is reflected from a surface and detected by the sensor. Therefore, it would be advantageous to provide means to only detect light having the same state of polarization as the laser, while filtering out the other undesired states of polarization.
Wire grid polarizers comprise parallel wires or similar lines of metal arranged at equal distances from one another in a plane that is oblique or perpendicular to the incident light. Electrons in the wires are driven in the direction of the electric field vector of an electromagnetic wave propagating through the wire grid. The unrestricted movement of electrons along the longitudinal extension of the wires makes the wire grid act as a mirror, whereas the movement of the electrons in the transverse direction is confined by the small width of the wires. Hence the vector component of the electric field vector that is parallel to the direction of the longitudinal extension of the wires defines a portion of the incident light that is essentially reflected, whereas the vector component of the electric field vector that is perpendicular to the direction of the longitudinal extension of the wires defines a portion of the incident light that is essentially transmitted. The electric field vector of the transmitted light is therefore perpendicular to the wires, and a linear polarization is thus obtained by the wire grid.
The sum of the width of one wire and the distance between adjacent wires is the pitch of the grid and corresponds to a minimal period of the regular lattice formed by the grid. The distance between adjacent wires is typically smaller than the wavelength of the radiation.
Wire grid polarizers are used in semiconductor devices. In a CMOS process, for instance, parallel metal lines forming a wire grid can be produced in the back end of line, where the components of an integrated circuit are electrically connected by a wiring including structured metallization layers embedded in an intermetal dielectric.
High-contrast grating-based reflectors are used as reflectors in advances vertical cavity surface-emitting lasers (VCSELs). These reflectors have the property that only one state of polarization is reflected thus fostering emission with one polarization state only. Moreover, high-contrast grating reflectors consisting of a single layer are easier to manufacture than the commonly use Bragg-reflectors, where about 30 to 50 layers are requires to achieve a meaningful reflection coefficient.
US 2007/0115553 A1 discloses a sub-wavelength grating structure for broadband mirrors and high-reflectivity gratings. The device comprises a substrate layer of silicon, a layer of low refractive index, like SiO2, and sections of a material of high refractive index, like polysilicon, spaced apart from one another and arranged on the layer of low refractive index.
US 2014/0353583 A1 and US 2014/0353530 A1 disclose polarization independent photodetectors with high-contrast gratings arranged above an air gap.
M. Guillaumée et al., “Polarization sensitive silicon photodiodes using nanostructured metallic grids”, Applied Physics Letters 94, 193503 (2009), present design, fabrication and characterization of wire grid polarizers with dimensions that are compatible with CMOS technology.
W. Hofmann et al., “Long-Wavelength High-Contrast Grating Vertical-Cavity Surface-Emitting Laser”, IEEE Photonics Journal 2, 415-422 (2010), present a VCSEL structure based on a subwavelength high-contrast grating reflector consisting of amorphous silicon on isolator as the output mirror.
T. Stöferle et al., “Ultracompact Silicon/Polymer Laser with an Absorption-Insensitive Nanophotonic Resonator”, Nano Letters 10, 3675-3678 (2010), for which supporting online material is available, present a planar nanophotonic resonator and a high-contrast grating mirror made from dielectric material as a subwavelength phase grating employed as an in-plane reflector in a laser cavity.
C. J. Chang-Hasnain and W. Yang, “High-contrast gratings for integrated optoelectronics”, Advances in Optics and Photonics 4, 379-440 (2012), give an overview of high-contrast gratings and the underlying physical principles.
Light absorption in the metal lines of a wire grid polarizer may reduce the responsivity of the sensor that is provided with the wire grid polarizer. The effect of the wire grid depends on the angle of incidence, owing to the height of the metal lines, and the angular polarization dependency may be relatively large resulting in tight tolerances. Furthermore, the extinction ratio of a state of polarization that is not to be detected is limited by the extinction coefficient of the metal.